A. Field of the Invention
This invention relates to central-station power-generation systems and, more particularly, to a combined Rankine/Brayton heat engine cycle.
B. Description of the Related Art
The Rankine-cycle, typified by a conventional steam engine, minimizes compression work by capitalizing on the energy exchange involved when a liquid changes to a vapor. Although Rankine-cycles can have higher thermal efficiency than other cycles because pressuring water is simple and does not consume much energy, transferring heat to the steam poses a problem. The necessity of using high-temperature, boiler-tube materials capable of containing the high-pressure working fluid limits the maximum possible superheated-steam temperatures to less than 1300.degree. F. Efforts to maximize the thermal efficiency of power plants employing a Rankine-cycle have plateaued at an efficiency of approximately 40%.
The Brayton-cycle, typified by a conventional gas-turbine engine, ideally consists of two constant-pressure (isobaric) processes and two adiabatic-reversible (isentropic) processes. In actuality, irreversible losses occur; therefore, no engine is capable of wholly isobaric or isentropic processes. In an ideal Brayton-cycle system, gas is compressed isentropically, heated at constant pressure, and then expanded isentropically through a turbine. In an open-loop Brayton-cycle system, cooling occurs in the open atmosphere. In a closed-loop Brayton-cycle system, cooling occurs in a heat exchanger.
In a Brayton-cycle system, the fuel consumes less than 100% of the air's oxygen content. This is because part of the air must be used for cooling, so that heat limits of the engine's materials are not exceeded. Present state-of-the-art metallic materials have heat limits of about 1800.degree. F. Present state-of-the-art ceramic materials have heat limits of about 2300.degree. F. Recently, carbon-carbon materials have been considered for use in Brayton-cycle, due to the ability of the carbon-carbon to retain tensile strength at high temperatures. A carbon-carbon turbine rotor, for example, was tested at temperatures of approximately 2800.degree. F. at 30,000 revolutions per minute in a stoichiometric-fueled turbojet-engine, as discussed in "LTV, Garrett Run Carbon/Carbon Turbine Rotor," Aerospace Propulsion, vol. 3, no. 5, p. 2 (Mar. 5, 1992) and in "LTV, Garrett Run Carbon/Carbon Turbine Rotor at 3000.degree. F.," Aerospace Daily, vol. 161, no. 39, p. 310 (Feb. 26, 1992).
A combination of the Brayton-cycle and the Rankine-cycle in series has been proposed to recover waste heat from the single Brayton-cycle system. This combined-cycle system uses the Brayton-cycle exhaust heat to heat the Rankine-cycle water/steam. Thus, the heat rejection of the overall system is from the Rankine-cycle and can be close to ambient temperature. In an integrated, combined-cycle system, part or all of the shaft power necessary to drive the Brayton-cycle compressor is supplied by the Rankine-cycle turbine. Combined-Brayton-Rankine-cycle systems produce higher efficiency than either system alone. However, these systems are complex since they essentially combine two thermodynamic heat engines into one powerplant system.
A combined Brayton-Rankine-cycle for a high temperature gas-cooled nuclear reactor is discussed in Powerplant Technology, by M. M. El-Wakil, on pages 350 and 351. In this system, the closed-loop Brayton-cycle is coupled to the Rankine-cycle via a feed-water heater. In the Brayton-cycle, helium is compressed in a helium compressor. The compressed helium is preheated in a regenerator then enters the high-temperature, gas-cooled nuclear reactor and emerges at a higher temperature. The helium then expands in a helium turbine and enters a regenerator. Remaining energy is transferred to the Rankine-cycle via a closed-type steam feed-water heater. The helium reenters the helium compressor.